In an article conveying and sorting system, such as that used by the U.S. Postal Service to convey and sort mail, individual letters are loaded into containers on the conveyor system according to their respective zip code destinations. The containers are then transported by air or truck to the destination and the letters are fine sorted and distributed to individual letter carriers for delivery to the respective addressees. To ensure efficient handling of such letter mail, the individual containers should be substantially filled to their respective capacities, which necessitates the use of containers having sufficient structural strength to withstand the weight of the letters contained therein and yet being sufficiently light for optimum handling and transportation of the containers. Furthermore, the containers should have the capability of preserving the pre-sort of the mail into groups (preferably by destination) during transit to the destination so that the individual letters are not intermingled.